Lactation pattern and the risk for hormone-related female cancer in Japan: the Ohsaki Cohort Study.

Although breastfeeding is associated with a reduction in the risk for breast cancer, its relationship with another hormone-related female cancer, endometrial cancer, has not been fully investigated. The objective of the present study was to prospectively examine the association between lactation pattern and the risk for incidence of breast cancer and endometrial cancer in Japanese women. We analyzed data for 26 680 women registered in the Ohsaki National Health Insurance Cohort Study, who were 40-79 years old at the baseline. During the 11 years of follow-up, we identified 148 incident cases of breast cancer and 32 incident cases of endometrial cancer. Compared with breastfeeding only, multivariate hazard ratios and 95% confidence intervals for the risk of breast cancer incidence were 1.12 (0.92-1.37) for women who had performed mixed feeding and 1.80 (1.14-2.86) for those who fed their babies only with formula (P-trend=0.014). For endometrial cancer incidence, multivariate hazard ratios and 95% confidence intervals were 1.32 (0.86-2.03) for women who had performed mixed feeding and 3.26 (1.23-8.61) for those who had performed only formula feeding (P-trend=0.018). Our findings appear to raise the possibility that nonbreastfeeding is positively associated with the risks for both breast cancer incidence and endometrial cancer incidence. Confirmation of our findings would require further investigation.